Eco-efficient control of the cooling systems for power transformers

Accepted Manuscript Eco-efficient control of the cooling systems for power transformers Lech Borowik, Rajmund Włodarz, Krzysztof Chwastek PII:

S0959-6526(15)01919-8

DOI:

10.1016/j.jclepro.2015.12.094

Reference:

JCLP 6567

To appear in:

Journal of Cleaner Production

Received Date: 29 December 2014 Revised Date:

5 October 2015

Accepted Date: 28 December 2015

Please cite this article as: Borowik L, Włodarz R, Chwastek K, Eco-efficient control of the cooling systems for power transformers, Journal of Cleaner Production (2016), doi: 10.1016/ j.jclepro.2015.12.094. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Eco-efficient control of the cooling systems for power transformers

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Lech Borowika, Rajmund Włodarzb, Krzysztof Chwasteka

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42-201 Częstochowa, Poland

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Abstract

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Operation of large transformers requires heat transfer by the cooling systems. An appropriate

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control of the cooling system by Programmable Logic Controllers allows one to decrease the ,,loss of

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life” of the insulation system of the transformer. Additional diagnostic functions aimed at improved

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asset management may also be easily implemented. Practical examples of eco-friendly solutions

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implemented at PPH Energo-Silesia Ltd. are presented and discussed in detail. Asset management of

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transformer system may include techniques aimed at heat recovery and noise reduction.

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Keywords

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power transformer, cooling system, noise reduction, asset management

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PPH Energo-Silesia Ltd., Opolska 21B, 42-120 Zawadzkie, Poland

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Faculty of Electrical Engineering, Częstochowa University of Technology, Al. Armii Krajowej 17,

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1. Introduction The concept of sustainable development has recently been the subject of intensive study worldwide

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(Bonilla et al., 2010, Despeisse et al., 2012, Duić et al., 2015, Giddins et al., 2002). The need to

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introduce improvements both to industrial processes and environment management systems aimed

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at better usage of resources and reduction of environmental impact has been recognized by the

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governmental agencies and the engineering community. Sustainable development is usually

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understood as a synergetic interaction between environment, society and economy, cf. Fig. 1.

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Fig. 1. Three pillars of the sustainable development

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European power engineering system faces at present a number of challenges related to practical

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implementation of European Commission directives (EC, 2007, EC, 2011) and ISO 14001 certification

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standard (Boiral et al., 2012, Nagel, 2003), aimed at improvement of environmental performance and

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reduction of emitted greenhouse gases. Deregulation of energy market, the increasing roles of

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renewable energy sources and emerging Smart Grid and Super Smart Grid networks (Cardenas et al.,

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2014, European Technology, 2006, Purvins et al., 2011) have redefined the paradigms of flexible and

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reliable energy distribution systems and the end-use consumers of electrical energy are becoming

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,,prosumers” (producers + consumers). The increasing demand for electric energy requires versatile

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and coordinated initiatives aimed at modernization and rationalization of power engineering policy.

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The fundamental actions undertaken to achieve this goal are: mastering of novel technologies to

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ACCEPTED MANUSCRIPT produce and transfer energy (Ferreira and Almeida, 2012), production of modern energy-saving

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electric machines using improved technologies (Cardenas et al., 2014, Ferreira and Almeida, 2012)

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and the attempts to recover, at least partially, loss dissipated in power engineering devices.

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Power transformers, whose aim is to transfer and distribution of electric energy, belong to the most

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important and – at the same time – most expensive components of the power engineering system

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(Kulkarni and Khaparde, 2004). Any emergency shut-down of a power transformer always results in

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serious perturbations related to breaks in supplying energy, what in turn results in losses

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experienced by the energy consumer. Therefore it is crucial to minimize the destructive processes

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occurring in the devices as well as to develop more reliable diagnostic methods in order to minimize

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the possibility of occurrence of emergency states (Lesieutre et al., 1997, Leibfried, 1998, Ristic and

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Mijailovic, 2012, van Schijndel, 2010). One of the methods to provide the optimum performance of

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power transformers is to maintain the proper temperature of their insulation system. In particular

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for larger units with rated apparent power of the order of several hundred MVAs carrying away the

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heat due to losses (reaching up to 2%) is a serious technical challenge. The losses may reach even

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several MW for biggest units, what is a substantial value and therefore any attempt to reduce it or

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recover part of it for other purposes is an ambitious and desired task.

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Yet another environmental problem related to the existence of large power transformers is the noise

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produced by these units (Borucki et al. 2011, Zając et al., 2011). The excessive noise is a burden for

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the people living in the neighborhood of transformer stations. It is necessary to assure safety zones

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between the power engineering stations and the municipal buildings, thus a significant part of plots

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of land, which could otherwise be inhabited, is wasted.

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2. The role of diagnostics and asset management techniques for power transformers

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Diagnostics and monitoring play an ever increasing role in contemporary industry, as they allow one

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to eliminate the possible sources of faults and may lead to substantial economic savings. Technical

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diagnostics is aimed at assessment the state of a technical device from the measurements of

ACCEPTED MANUSCRIPT diagnostic signals (Zakrzewski, 2012). This branch of knowledge is interdisciplinary and combines a

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number of concepts from control science, metrology and computer science (Borowik, 2003). The

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following fundamental tasks of diagnostics may be distinguished:

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- examination, identification and classification of failures and their symptoms,

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- development of methods and measures to examine and select diagnostic symptoms,

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- generation of diagnostic decisions based on the state of the examined device and taking necessary

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precautions to avoid failure, damage or loss.

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A classification of methods of failure detection during industrial processes is depicted in Fig. 2.

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As it can be seen, the methods may be classified into two fundamental groups:

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- methods based on control of parameters of process variables

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- methods based on control of relationships between process variables

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Fig. 2. Classification of the methods of failure detection (Borowik, 2003)

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The first group relies on measurements and analysis of time variations for individual diagnostic

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signals. The values of process variables are controlled and compared to the so-called threshold

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values without carrying out an analysis of relationships between them. The diagnostic methods

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belonging to the first group are relatively simple, as they do not require the knowledge provided in

ACCEPTED MANUSCRIPT the form of the models of processes or devices. They are often reliable and robust enough to be used in practice. However the major disadvantages of these methods are: the limited amount of supplied information and their ambiguity, which happens quite often (the same diagnostic signals may result from different types of failures).

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Another group of failure detection methods makes use of the existing relationships between process variables. The methods belonging to this category require accurate knowledge on the monitored device or process. These methods are fundamental in contemporary technical diagnostics. Recent

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advances in computer science have made it possible to apply these methods in the online mode. A computer system allows one to ,,record’’ both the formal knowledge expressed in the form of

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models and mathematical relationships, as well as the pieces of information acquired from the personnel.

The concept of ,,asset management” is in a close relationship with the roles to be played by the diagnostic system. In management science ,,assets’’ are defined as physical plants, devices,

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machineries and other items that have a distinct and quantifiable business function or service. The notion ,,asset management’’ denotes systematic and coordinated activities and practices through which an organization optimally and sustainably manages its assets and asset systems, their

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associated performance, risks and expenditures over their life cycles for the purpose of achieving its organizational strategic plans (PAS 5-1, 2008, PAS 55-2, 2008, Schneider et al., 2006, van Schijndel, 2010). A correctly prepared assess management strategy allows one to provide the optimum working

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conditions for the power transformer (avoidance of failures, scheduling of planned shut-downs for maintenance, assessment of ,,loss of life” factor) (Abu-Elanien et al., 2010, Velasquez-Contreras et al. 2011, Zhang and Gockenbach, 2008). 3. A brief introduction to problems related with cooling systems for transformers In order to distinguish easily the kind of cooling used in a given transformer, international coding has been introduced in order to determine both the medium and the circulation mode for the internal

ACCEPTED MANUSCRIPT part, which directly takes the heat from the active components and the external part, which takes

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the heat from the internal system. Figure 3 provides information on the notation.

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Fig. 3. Standardized method for notation of cooling systems in transformers

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The code for cooling method consists of four letters, except for dry transformers, as these possess

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just one medium for heat transfer. The first letter denotes the kind of the internal medium: A – air, O

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– oil, G – gas (e.g. SF6). The second letter denotes the method to put the medium into motion: N –

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natural (convection), F – stimulated by a fan or a pump, D- stimulated and at the same time directed

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through channels in the windings. The third letter denotes the external medium: A – air, W – water.

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The fourth letter denotes the method to put the external medium into motion: N – natural

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(convection), F – stimulated by a fan or a pump.

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The most common types of cooling systems are OFAF and ODAF systems, i.e. those with stimulated

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oil circulation and fan controlled air circulation. An in-depth analysis of the oil-forced and oil-directed

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cooling of power transformers has been provided in the paper by (Sorgić and Radaković, 2010).

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Photographs 4a and 4b depict two common solutions:

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Fig. 4. Two common solutions for OFAF systems: Fig. 4.a. Coolers mounted directly on

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transformer tank, Fig. 4.b. Coolers make up the so-called cooling battery, which is located next to

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the transformer

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Stimulated oil and air convection intensifies to a large extent the thermal capacity of the

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cooling system, what makes it possible to reduce the volume and dimensions of the device. This

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is particularly important due to transportation demands, the volume of transformer oil, the mass

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of the device etc. Unfortunately, the temperature distribution along the transformer height is not

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uniform, what results from the non-uniformity of oil streams inside the transformer. Most of the

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oil flows in the region between the tank jacket and the windings. In the top part of the

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transformer all oil streams mix together, thus the temperature distribution inside the windings is

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significantly different both in the lower and the upper parts of the transformer, what in turn is

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the reason for increased thermal stresses, cf. Fig. 5.

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Fig. 5. The OFAF cooling system with marked distribution of oil streams

Q

The total oil stream

Q  Q

flowing through the transformer consists of a larger number of

 QGN  QDN  QR  Qi , where QK - oil flowing between the active part

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streams:

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and the tank wall [m3/s], QGN and QDN - oil flowing between the cooling channels of the upper

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and the lower voltage winding, respectively, [m3/s], QR - oil flowing through the channels cooling

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the core, [m3/s], Qi - oil flowing through the channel of main insulation, [m3/s]. Also the total

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power losses of the transformer may be divided into terms related to oil streams:

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 P  P

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are power losses in the upper and the lower voltage winding, respectively, whereas PR are the

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power losses in the transformer core.

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K

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 PGN  PDN  PR , where PK are power losses in the tank, PGN and PDN

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In order to determine the average rise in temperature of a winding Qmw with respect to ambient temperature, the following coefficients are introduced:

an 

Pn - the relative power transferred to the n-th stream  P

ACCEPTED MANUSCRIPT Qn - the relative outcome of n-th oil stream. Q

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wn 

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Thus the average rise in the upper voltage (GN) winding is given with the expression:

  a mwGN  c   0.5 GN  1 DC , where c is the temperature rise in the top part, [oC], wGN  

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aGN is the coefficient of relative power transferred to the upper voltage winding, wGN is the

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relative outcome of the oil stream related to upper voltage winding, DC is the difference of

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temperatures and the inlet and the outlet of the cooling system, [oC]. In a similar way one

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calculates the average rise of oil temperature in the area of lower voltage winding.

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It has been remarked in technical reports that the oil outcome flowing directly through the

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windings does not depend much on the total oil outcome stimulated by the pumps. This is

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explained by the existence of the drain trap phenomenon, like in the case of natural convection.

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In practice, the higher is the total oil outcome (the number of working pumps), the lower is the

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relative oil outcome flowing through the windings. The respective values are estimated

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empirically on the basis of several thermal loading tests of different transformers with rated

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power above 100 MVA. Such a method of determination of the average rise in oil temperature is

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proposed in the international standard IEC 60076-2. It should however be remarked that in

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practical calculations concerning temperature rise in the system winding/oil, the average oil

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temperature is determined from the measured temperature values at the inlet and the outlet of

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the cooling system.

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Thermal capacity of the cooling system may be further increased applying a directed oil flow

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though the winding interior. Using specially designed barriers the oil flow is stimulated in the

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channels in the horizontal direction, there is no possibility for the oil to flow between the tank

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and the active part, what is common for the OF systems. A simplified sketch of oil flow in the

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ODAF system is presented in Figure 6.

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Fig. 6. A simplified sketch of oil flow in the windings for the ODAF system

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In this solution oil is pressed into the chamber below the windings, then it flows out to

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individual columns and windings. Such system geometry suppresses an intensive flow in the

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horizontal channels, what results in the increase of heat transfer coefficient from the windings

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and makes the temperature distribution in the windings more even. The volume of the channels

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and the partition into sections in the individual windings are designed in such a way, so that the

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oil flow in the individual sections was proportional to power losses, what is affected by the

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hydraulic resistances of individual flow streams. Their parallel connection assures the demanded

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partition of oil streams. A solution is possible, in which for each column there is a separate

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chamber. The total oil outcome for each column is given with the formula Qolf  0.75

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where

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nk is the number of windings in the column. The 25% margin in the above-given formula is

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related to the fact that a part of oil flows directly to the tank are, thus it surpasses the windings.

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An important parameter is the oil velocity in individual channels. This value should not exceed

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1 m/s due to the possibility of occurrence of static electrification. Oil temperature rises are

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Q

ol

nk

,

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Q

ol

is the total oil outcome dependent on the type and the number of pumps, [m3/s],

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calculated from analogous formulas like for the OF system, whereas the coefficients a and w are

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determined from the relationships a 

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The most important differences between the OFAF and the ODAF systems may be summarized as follows: -

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3Qolf Pu and w  . 0.75 Qol  P

OD-type cooling assures a more uniform temperature distribution in the windings, what results in the assumption of higher values of admissible temperature rise in the system

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winding/oil. According to international standards the respective values are 70 oC for the

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OD-type system and 65 oC for the OF-type system. -

For OD-type cooling different control procedures for the number of working cooling

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systems are needed. Above all it is crucial to eliminate the possibility to switch off all the

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pumps.

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In the OF-type cooling it is possible to switch off temporarily all the pumps, monitoring the oil temperature level in the top part.

4. Practical eco-friendly solutions implemented at PPH Energo-Silesia, Poland

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PPH Energo-Silesia Ltd., the employer of one of the authors, is a medium-sized enterprise operating

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in the market of electrical power engineering in southern Poland. Its mission is to promote eco-

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friendly designs of power transformers and auxiliary devices, with particular attention paid to cooling

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systems (Borowik and Włodarz, 2011, Borowik et al., 2010, Zając et al., 2011). Innovative solutions

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implemented at Energo-Silesia Ltd. include:

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- development of novel monometallic double-finned pipes used in the transformer oil coolers,

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- the possibility to use of Programmable Logic Controllers (PLCs) to control the amount of heat

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generated in power transformers and to recover a part of heat for self-heating purposes of the

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power station,

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- reduction of noise level generated by cooling system fans using appropriate control algorithms.

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ACCEPTED MANUSCRIPT 4.1 Innovative design in the mechanical design of coolers

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The main requirements to be met by the mechanical construction of a cooler is that it has to suit

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various types of transformers: it has to be tight (this is rarely the case in the older types of coolers),

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the noise generated has to be low, the maximal oil temperature in the top layer cannot exceed 85°C;

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the ambient temperature may reach 40°C for extended periods. All these requirements are met by a

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cooler manufactured by Energo-Silesia Ltd. (Pasierb and Szajding, 2006). The construction includes

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aluminium pipes finned both on the inside and outside, as shown in Fig. 7.

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Fig. 7. Aluminium double-finned pipe (Pasierb and Szajding, 2006)

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Double finned pipes like those shown in Fig. 7 may be successfully used either in OFAF or ODAF

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systems discussed in the previous section. Exemplary dependences of heat transfer coefficient K on

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oil flow delivery for a finned and a pipe with smooth interior are shown in Fig. 8. It can be noticed

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that the respective K values for a double-finned pipe are approximately three times higher than for a

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pipe with smooth interior (sometimes described as a single-finned pipe).

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Fig. 8. A comparison of thermal efficiency for a double-finned pipe and a single-finned pipe

ACCEPTED MANUSCRIPT 4.2 Enhanced use of PLCs in cooling systems for diagnostic purposes

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One of the crucial elements of the cooling system is the controller. Since its built-in control

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algorithms determine the functioning of the whole system, a considerable attention should be paid

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to its proper choice. The most flexible scaling solution is offered by Programmable Logic Controllers.

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The main advantage of PLCs over compact cooling system controllers lies in their module

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construction, which makes it possible to obtain an optimum configuration.

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During normal operation of a transformer, the controller of the cooling system must respond to a

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number of interference signals and faults, such as power supply decay or asymmetry, operation of

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thermal protection system of one of the drives, oil pump breakdown, fan breakdown, fouling of

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coolers, or other adverse conditions affecting heat transfer. Control algorithms can cope with these

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and many more problems, since PLCs are capable of diagnosing a threat and taking measures to

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prevent negative effects.

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The use of a PLC as the central control unit in the cooling system of the transformer makes it possible

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to carry out additional diagnostic functions, as these devices have redundant computational

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capabilities. Noticing the fact that only a fraction of time is spent on calculations in the PLC has led to

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a novel approach implemented in PPH Energo-Silesia Ltd., which better utilizes the PLC resources for

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instant monitoring and control of the cooling system.

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Figure 9 depicts schematically most of the auxiliary functions implemented in PLCs.

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It should be remarked that practically all most important components that require monitoring and

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supervision are controlled, thus the diagnostic signals are available instantly. This knowledge may be

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very useful for asset management strategy and for taking preventive actions in order to avoid

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failures. The priority in the diagnostics system is given to monitoring of temperature in the so-called

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HOT-SPOTS, which are the hottest points of the coil. Any temperature rise above the admissible

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values is instantly detected and signaled as an emergency situation. The development of appropriate

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diagnostic methods and equivalent models supporting asset management strategies remains one of

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ACCEPTED MANUSCRIPT crucial problems for power transformer designers and engineers (Lesieutre et al., 1997, Radaković et

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al., 2014, Radaković et al., 2015, Susa and Nordman, 2009, Taghikani and Gholami, 2009).

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It should however be recalled that the role of the developed system is supplementary to the existing

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(and required) protection systems and devices, such as e.g. the Buchholtz relay. The role of the PLC-

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based system is to control the cooling of transformer in such a way, so that the critical values of

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parameters were never reached. In other words the PLC system should prevent abrupt variations of

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the monitored signals.

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Fig. 9. Diagnostic functions implemented in a cooling system controller (Borowik and Włodarz, 2011)

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Figure 10 depicts schematically a sketch of a controller that makes it possible to monitor and adjust

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thermal power in a continuous way. The system includes two-fan coolers, each of them is equipped

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with oil temperature sensors at inlet and outlet, as well as with an oil flow sensor.

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Fig. 10. A sketch of the cooling system controller developed in PPH Energo-Silesia Ltd.

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The signals from the sensors monitoring the quantities: the ambient temperature, the load current of

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the transformer and the parameters of angular speed of the fans, are forwarded to the cooling

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system controller. The system is capable to work using either external or backup power supply. In

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emergency situations the control system is automatically shut down.

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A more detailed Figure 11 depicts the block diagram of the cooling system controller developed by

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the authors (Włodarz and Borowik, 2013). The model has been verified on a physical installations and

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at present an invention application concerning the solution has been filed to the Polish Patent Office

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(registered as No. P.404036).

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ACCEPTED MANUSCRIPT Fig. 11. A block diagram of the cooling system controller developed by the authors. Abbreviations:

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AS/BS – analogue/binary signals, CPU – central processing unit, RSC – control of rotational speed,

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CC-1st, CC-nth – control of the 1st and the n-th cooler, respectively

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Practical positive assessment of the proposed approach has been proven by an experimental setup

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put into operation in November 2013 at power station RPZ Służewiec in Warsaw, Poland. The most

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important data concerning the transformer equipped with the prototype cooling system are as

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follows: rated power 80/40/40 MVA, 115 kV  10% (8 steps)/15.75 kV/15.75 kV, YNd11d11, OFAF

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cooling system. Preliminary tests at the pilot installation are very promising and have proven the

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effectiveness of the proposed solution.

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4.3. Waste energy recovery

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An average life time of a power transformer is several tens of years. The life time of transformer

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cooling systems is much shorter. Therefore during exploitation period of the whole transformer, the

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overhauls and modernizations of their cooling systems are carried out at least several times, what

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makes an excellent opportunity to introduce modern solutions. Such a situation took place during an

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overhaul carried out by PPH Elektro-Silesia in 2009. The rating parameters of the modernised

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transformer are given in Table 1. The total loss of the unit is about 112 kW. On the basis of data

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provided by the exploitation service the load of a single transformer during winter time was

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estimated at 4050% of the rated high voltage (HV), thus the level of power was 4050 kW per

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transformer. With this data treated as guidelines, a system was designed and constructed, as shown

290

in Fig. 6.

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ACCEPTED MANUSCRIPT 293

Table 1. Rated technical data of a transformer Rated power Rated power Rated power Voltage of Voltage of Voltage of Idle loss of HV of MV HV MV LV of LV winding winding winding winding winding winding

10 MVA

10 MVA

110 kV ± 16%

6.6 kV

25.8 kW

86.7 kW

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16.5 kV ± 2x2.5%

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16 MVA

Load loss

Fig. 12. Functional diagram of the developed system

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297

Taking into account the efficiency of the heat exchanger and noticing that much of the heat was

298

dissipated directly by the transformer tank to the atmosphere, it was concluded that the amount of

299

recovered energy would be sufficient to heat a building with the total cubature of 1640 m3 and

300

providing the minimal admissible temperature of +5C in the control room. An economical analysis of

301

the design has revealed that the radiators might be replaced by forced oil circulation coolers at the

302

additional expense (some 45%). The advantages are much better operating conditions for the

303

transformer i.e. the fulfilment of the ultimate goal of the modernisation. The recovery of some

ACCEPTED MANUSCRIPT amount of energy, which is usually wasted, becomes an additional benefit. The functions of the

305

cooling system and of heat recovery are correlated, so they may be monitored with the same

306

controller.

307

The analogue and binary inputs of the controller receive a number of process variables. One of them

308

is temperature at various points of the transformer, e.g. the top layer oil temperature, which is of

309

fundamental importance for the cooling control system, since it determines whether an additional

310

cooler is to be switched on, or the whole cooling system is to be switched off. In the latter case, the

311

building is heated in the ordinary way, i.e. by means of electric heaters. The fact that various signals

312

representing the overall condition of the system are accumulated in one unit, i.e. the PLC, makes it

313

possible to perform additional algorithms, aimed at optimal control, diagnostics, and monitoring.

314

Since the potential faults are diagnosed quickly, this piece of information can be immediately sent to

315

the dispatcher by the remote control unit. If necessary, the system can enter the so-called safe state,

316

during which the breakdown can be eliminated without switching off the transformer. The heat

317

recovered from the transformer is used in three rooms: the 6 kV switching station with two heaters,

318

the control room with one heater, and the remote control room with a convection heater. During the

319

next stage of modernisation, the heat recovery system will be supplied for transformer 2 and heaters

320

will be placed in the 15 kV switching station room. In order to verify the theoretical assumptions of

321

the project, selected temperature values were recorded for a period of time, as depicted in Fig. 13.

322

It can be noticed that the condition of maintaining the minimal temperature at +5C (relevant for

323

control room "3") was met, despite a large variation of ambient temperature. There was no need to

324

switch on another type of heating.

325

Excess heat was directed to the 6 kV switching station room, where temperature above 0C was kept

326

during the whole analysed period, even though the condition of thermal insulation of this room was

327

rather bad.

328

The amount of energy used for heating the rooms depends on a number of factors, among which the

329

transformer load and ambient temperature are the most important: the transformer load

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ACCEPTED MANUSCRIPT determines the amount of energy lost and recovered, whereas the ambient temperature determines

331

how much heat is dissipated directly from the transformer tank. However, regardless of the specific

332

conditions of load and temperature, it has to be noted that the amount of energy recovered is

333

significant and the effort invested in the recovery process is always profitable.

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334

Fig. 13. Ambient temperature and minimal daily temperature attested inside the rooms

336

(data for December 2009 – February 2010)

337

Figure 14 presents daily amounts of energy received from the transformer and supplied to the

338

building in the period under consideration.

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340

Fig. 14. Daily amounts of thermal energy, [kWh/day], received from the transformer in the period

341

December 2009 – February 2010

342

ACCEPTED MANUSCRIPT 4.4. Noise reduction in the cooling system

344

The fans in the cooling system of a power transformer are the source of substantial noise. Even

345

though the reduction of the noise level is not as essential task as transformer cooling and heat

346

recovery, the cooling system can be controlled in such a way that noise is lowered below the

347

permissible values given in the standards (Legislative decree, 2004, Polish legislative act, 2001). The

348

research carried out at PPH Energo-Silesia Ltd., focused on design and construction of a low-noise,

349

high-efficiency fan cooler, has led to the development of a cooler equipped with axial fans from

350

Ziehl-Abegg (Ziehl-Abbeg, 2011), featuring aerodynamic bionic profile. The number of pipe rows

351

could be reduced, as the heat exchange was more efficient in comparison to the typical approach.

352

Therefore, for a constant air supply, the fans could operate at lower pressures.

353

The relationships between air supply, pressure and noise generated by a fan, type FC080-6D_.6K.A7

354

are depicted in Fig. 15. The dependencies were plotted for various supply voltages from the range

355

140V - 400V and corresponding rotational speeds from the range 430 rpm– 900 rpm. The introduced

356

design modifications and the use of modern fans brought about noise reduction by 2 - 4 decibels.

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357

ACCEPTED MANUSCRIPT Fig. 15. Dependencies of pressure on air supply for different fan settings

359

Another method of noise emission reduction is the optimisation of rotational fan speeds. This

360

method is currently being developed and a pilot system has been installed in one of the major power

361

engineering companies in Poland. Rotational speed can be adjusted in many ways, e.g. with multi-

362

gear motors or by voltage control. Preliminary results indicate the benefit from using a frequency

363

converter for this purpose.

364

The dependence of noise on rotational speed for a single cooler is presented in Fig. 10. As there are

365

several coolers in the transformer cooling system and each of them has several fans, the concept is

366

to control coolers and fans in such a way that thermal power required for cooling the transformer is

367

maintained but – on the other hand – noise is minimised.

368 369

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Fig. 16. Dependence of noise on rotational speed for a single fan

370

As it has already been mentioned, the operation of the whole system is controlled with a single PLC.

371

In the basic operating mode, the frequency converter is on, but it can be switched off on demand of

372

the service team or in case of emergency. Then the cooling system starts operating in the

373

conventional mode. Each of the fans has its own protection and a switch, so the number of fans

374

operating at any moment may be determined by the control algorithm. In the operating mode with

375

the rotational speed adjustment the most advantageous solution is to switch the maximum number

ACCEPTED MANUSCRIPT of fans on. Considering the noise level, it is advisable to let the fans be positioned non-centrally, and

377

at lowest locations to let them work at higher rotational speeds.

378

This method of noise reduction is very promising, what is confirmed with the results from the pilot

379

system. The cooling system of each transformer is designed in such a way that it can meet the

380

requirements of working under extreme conditions, such as overload, high ambient temperature,

381

etc. In practice, such extreme conditions are not met frequently. Moreover at night, when the

382

permissible noise level is lower, temperature drops and the cooling conditions are improved.

383

Therefore the cooling system has a margin of thermal power at its disposal, which can be utilised for

384

the purpose of noise reduction. The effective reduction of acoustic pressure obtained by applying

385

smooth control to the rotational speed of fans can be appreciated from Fig. 17.

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Fig. 17. Noise level versus thermal power of the cooling system

388

The presented solution has yet another feature, important in emergency situations. Then the cooler

389

might have to absorb thermal power higher than rated. In the conventional designs this could be

390

impossible. In the smooth control mode it is possible to increase the speed by 25% over the rated

391

speed, which, in turn, yields 120% of the rated thermal power. The PLC is configured to handle this

ACCEPTED MANUSCRIPT case as well. The increase in rotational speed results in an increased noise level, nonetheless the

393

proper cooling conditions for the transformer are of paramount importance.

394

From the measurements it follows that taking into account the dependence noise level versus

395

thermal power of the cooling system it is most advantageous to run the maximum available number

396

of coolers at a given time instant. Theoretical assumptions and measurements have been verified on

397

a real-life unit. The Company RWE Stoen Operator, the administrator of electrical power engineering

398

network in Warsaw has given a permission to install a prototype system. The rated parameters of the

399

transformer used for prototype testing are listed in Table 2.

400

Table 2. The rated parameters of the prototype transformer.

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Rated voltages

High voltage winding

115 kV  10%

TE D

Low voltage winding I/II 15.75/15.75 kV Rated power

40/20/20 MVA

Idle run current

 0.5% In

Connection scheme

YNd11d11

Cooling system

OFAF

Idle run losses

 20 kW

Load losses

230 kW

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401 402

The cooling system for the transformer consisted of a set of two oil-air coolers, CHOPN-200 type.

403

Each cooler is equipped with a pump, CTR-125-5.5 type, and two fans, both FC080-SDD.6K.3 type.

ACCEPTED MANUSCRIPT Taking into account compact settlement density and relatively small distances between habitable

405

buildings and power stations, the problem related to excessive noise level is in Warsaw particularly

406

important. The prototype solution has been tested for more than a year; in the meantime the

407

measurements of acoustic pressure corrected in accordance with frequency dependence type A. The

408

noise level for fans in the on and off states have been measured in the points distant 2 m away from

409

the main emission surface. The principles for carrying out such measurements are defined in the

410

standard PN-EN 60076-10 (Polish Standard, 2003), the location of measurement points is depicted in

411

Fig. 18. 12

13

14

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17

16

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19

Low voltage side

fan

cooler

L1

L2

2m

6

4

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5

L2

21

L3

L3

22

23 control rack

TC

7

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L1

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8

L3

fan

L2

L1

9

cooler

N

20

oil expansion chamber

10

24

high voltage side

3

2

1

27

26

25

413

Fig. 18. Setting of measurement points around the transformer (RWE Stoen Operator)

414

Taking into account the height of transformer tank h > 2.5 m, the measurements of acoustic pressure

415

have been carried out for two horizontal measurement lines. The first measurement line was

416

defined for the height h1 = 1.5 m, the other one for the height h2 = 3.0 m. The measurements of

417

corrected level of acoustic pressure for the background have been carried out in the 110 kV switching

418

station, where the examined transformer had been installed. The measurements have been carried

419

out for seven states of work of the cooling system, i.e.:

420

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

fans off, pumps off



fans off, pumps on

422



fans on, 20% of rated speed, pumps on

423



fans on, 40% of rated speed, pumps on

424



fans on, 50% of rated speed, pumps on

425



fans on, 60% of rated speed, pumps on

426



fans on, 80% of rated speed, pumps on.

SC

421

In Table 3 the average corrected (in accordance with the frequency dependence, type A) value of

428

acoustic pressure of the transformer, determined from measurements for different states of the

429

cooling system, is presented. On the basis of the carried out analysis it can be stated that for rated

430

work conditions of two oil pumps and operation of two coolers, up to 50% of the rated speed of their

431

fans (the motor windings in the star connection) there is no significant increase in noise level of the

432

transformer unit if compared to the noise generated by the unit with the cooling system shut down.

433

The average values have been calculated for all measurement point at given height, in accordance

434

with the guidelines of the standard PN-EN 600076-10 (Polish Standard, 2003). In this specific case it is

435

easier to assess the effectiveness of the proposed control method for the cooling system if only the

436

measurements for the points located right in front of the coolers are considered, as for these points

437

the most noticeable effects are expected. The obtained results are well illustrated in Fig. 19, where in

438

the circular diagram the distribution of the acoustic pressure in individual measurement points is

439

shown for individual operation modes.

440

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

cooling system determined for different heights and different cooling conditions

Operation mode

fans off, pumps off

67.5

fans off, pumps on

67.6

fans on (20 % of the rated speed), pumps on

67.4

fans on (40 % of the rated speed), pumps on

67.5

fans on (50 % of the rated speed), pumps on

67.6

fans on (60 % of the rated speed), pumps on fans on (80 % of the rated speed), pumps on

Acoustic power of the transformer unit together with the cooling system, dB

67.4

89.29

67.5

89.41

67.7

89.53

67.4

89.29

67.5

89.41

68.6

68.7

90.66

70.0

70.2

92.31

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Corrected level of acoustic pressure at 3.0 m, dB

RI PT

Corrected level of acoustic pressure at 1.5 m, dB

SC

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Table 3. Corrected levels of acoustic pressure and acoustic power of the transformer unit with the

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444 445

Fig. 19. The results of measurements of acoustic pressure, dB, corrected

446

in accordance with the frequency dependence, type A.

ACCEPTED MANUSCRIPT Analysing the obtained results, special attention should be paid to measurement point 6-10 and

448

20-24. In these areas the increases of the acoustic pressure level, corrected in accordance with the

449

frequency dependence, type A for measurements carried at 50% and 80% of the rated speed are

450

about 5 dB. The fundamental parameters concerning the work of the transformer have been and are

451

still monitored on-line and archived every 10 minutes. The data acquisition is carried out in an

452

uninterrupted way since 31st October 2012. In the examined period the necessity to use the fans with

453

the speed exceeding 50% of the rated speed has never occurred so far. In the hottest period (July

454

and August 2013) the maximum speed of the fans did not exceed 42%. For other months, taking into

455

account lower ambient temperatures, the cooling conditions were more advantageous.

456

Summing up to this point it can be stated that noise reduction with the method based on smooth

457

control of the rotational speed of the fans brings significant advantages, as it allows one to reduce

458

the noise level to a large extent. It follows from the fact, that in every case the cooling system of the

459

transformer is designed taking into account its maximum losses and possible overload and for worst

460

possible ambient conditions. It can be stated that the proposed method makes it possible to avail of

461

the reserves being at the disposal of the cooling system.

462

The problem of excessive noise is more and more acute in big cities, what is caused by ,,approach’’

463

of households to electrical power stations. The example of Polish capital, Warsaw, is a paradigm for

464

this issue. The fulfillment of the requirements on admissible noise level resulting from legislative acts

465

issued by the Ministry of Environment Protection is becoming a challenge for electrical power

466

engineering.

467

5. Conclusions

468

In the paper an innovative design of the cooling system of power transformers is presented. It is

469

shown that an improvement of the algorithm implemented in the Programmable Logic Controller

470

controlling the cooling system leads to better asset management and is an environment-friendly

471

solution (e.g. allows one to recover a part of generated waste energy for heating purposes).

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ACCEPTED MANUSCRIPT Contemporary Programmable Logic Controllers have redundant computing power, which may be

473

used for technical diagnostics and monitoring. The crucial components of the power transformer and

474

its auxiliary systems may be examined and the obtained data may be used for planning maintenance

475

checks and overhauls. Moreover it is possible to reduce the noise level generated by fans in the

476

cooling system by an appropriate ,,smart’’ control algorithm.

477

The proposed concept allows one to reduce both the economic costs related to unexpected failures

478

of the power transformer and the impact on environment. Therefore it may be an example of

479

sustainable development of the electric power engineering. The idea may be generalized to any type

480

of production and manufacturing: it is crucial to reduce waste on the spot at the production place by

481

a tailored use of resources instead of debating what to do with the waste later. This approach is

482

consistent with contemporary trends worldwide, as discussed with this journal, cf. e.g. the paper by

483

(Zeng et al., 2014).

484

It should be remarked that the benefits from the implementation of the approach presented in the

485

paper are at the moment hard to be evaluated and generalized. In the paper the results obtained at a

486

pilot installation are presented. In order to make a thorough analysis it is necessary to wait for the

487

effects for a number of years. The paper proves that there is a possibility to recover heat e.g. for

488

heating the places where tele-mechanical devices of the switching gear unit are located and to

489

reduce the noise level.

490

As a rule the ,,life” of a cooling system is much shorter than that of the power transformer. Therefore

491

there is a regular need to carry out overhauls and maintenance works on the cooling systems. At

492

little additional expense the systems may be modernized and fulfill additional diagnostic tasks. It

493

seems not feasible to work out a single universal rule of thumb for the design of the cooling systems.

494

In any particular case the location of the power transformer and the object to be heated with the

495

recovered heat should be analyzed individually. On the other hand this paper stresses the fact that

496

in the era of depleted fuel resources any primary fossil and energy source should be preserved.

497

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wiedzy

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wybranych

urządzeń

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L.,

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proposal for a novel solution in the design of transformer oil coolers – in Polish), proceedings of VI

558

Scientific-Technical Domestic Conference ,,Power and special transformers”, Kazimierz Dolny, Poland,

559

pp. 169-180.

560

Polish legislative act ,,Environment protection law” from 27 April 2001 (announced in Dz. U. (Journal

561

of Laws of the Republic of Poland) No. 62 pos. 627 with subsequent modifications).

562

Polish Standard PN-EN 60076-10 – Transformers, part 10: determination of noise levels, Warsaw,

563

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564

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ACCEPTED MANUSCRIPT

Programmable Logic Controller may perform additional diagnostic tasks. Some part of waste energy may be recovered for heating purposes.

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Appropriate control may reduce noise produced by power transformers.